Structural Characterization of β-Agostic Bonds in Pd-Catalyzed Polymerization

Xu, Hongwei; Hu, Chunhua T.; Wang, Xiaoping; Diao, Tianning

doi:10.1021/acs.organomet.7b00666

Cited by 29 publications

(23 citation statements)

References 48 publications

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“…4–5 kcal/mol lower than the Pd(II) catalysts, accounting for the very significantly higher turnover frequencies in the nickel systems. (3) The branching densities and architectures of the polyethylenes formed are greatly affected by the behavior of the agostic alkyl intermediates ( 2 , 4 , 6 ). − These species undergo “chain-walking” via β-hydride elimination, olefin rotation, and reinsertion, one cycle of which moves the metal one carbon down the chain. NMR studies show the barrier to a 1,2 shift in Ni agostic intermediates is ca.…”

Section: Development Of Ni/pd α-Diimine Complexes For Ethylene Polyme...mentioning

confidence: 90%

Exploring Ethylene/Polar Vinyl Monomer Copolymerizations Using Ni and Pd α-Diimine Catalysts

2018

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The most ubiquitous polymer, polyethylene (PE), is produced either through a radical-initiated process or, more commonly, through a coordination/insertion process employing early transition metal catalysts, particularly titanium- and chromium-based systems. These oxophilic early metal catalysts are not functional-group-tolerant and thus cannot be used to synthesize copolymers of ethylene and polar vinyl monomers such as alkyl acrylates and vinyl acetate. Such PE copolymers have enhanced properties relative to PE and are made through radical polymerization processes, requiring exceptionally high pressures and temperatures. Copolymerizations of polar vinyl monomers with ethylene using more functional group-tolerant late metal catalysts potentially offer an attractive alternative for generating such value-added copolymers since ligand variations may provide more control of polymer microstructures and milder reaction conditions would apply. This Account describes our efforts, particularly through detailed mechanistic studies, to probe and develop this potential using Pd(II) and Ni(II) α-diimine catalysts. To inform discussions of the copolymerizations, we briefly review key aspects of ethylene homopolymerizations using these diimine catalysts. These include ligand designs that incorporate axial blocking groups that retard chain transfer and promote production of a high polymer rather than an oligomer. These ligand designs also lead to unique branched polyethylenes via migration of the metal along the chain ("chain-walking") prior to insertion. Mechanistic investigations of copolymerizations of ethylene with polar vinyl monomers using the diimine complexes have revealed several impediments to developing practical catalysts: (1) The polar group of the comonomer can coordinate strongly to the metal center, blocking coordination of ethylene. (2) Weak binding affinity of the polar monomer relative to ethylene can result in very low levels of comonomer incorporation. (3) A metal alkyl chain bearing a heteroatom, X, on the β-carbon atom can undergo β-X elimination leading to deactivation of the catalyst. (4) Stable chelate formation following insertion of a polar comonomer can greatly retard the rate of chain growth. (5) A metal alkyl chain bearing an electron-withdrawing heteroatom at the □-carbon atom can result in a high insertion barrier. A patent disclosure by the DuPont Versipol group and our extensive mechanistic studies reveal that, remarkably, vinyl trialkoxysilanes are ideal comonomers and circumvent all of the impediments noted above. The Pd-catalyzed copolymerization of vinyl trialkoxysilanes with ethylene produces highly branched, low molecular weight copolymers with activities comparable to those of analgous ethylene homopolymerizations. A 1,2- insertion of the vinyl silane results in the formation of a five-membered Pd-O(R)Si chelate which is readily opened by ethylene and thus does not reduce the rate of chain growth. β-Silyl elimination results in chain transfer and accounts for the lower molecular weight p...

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Section: Development Of Ni/pd α-Diimine Complexes For Ethylene Polyme...mentioning

confidence: 90%

Exploring Ethylene/Polar Vinyl Monomer Copolymerizations Using Ni and Pd α-Diimine Catalysts

2018

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“…The lower electronegativity of Ni results in a weaker agostic interaction relative to Pd and the smaller radius of Ni leads to a more strained geometry in the transition state. Both factors contribute to slower (b-H) elimination of Ni-alkyl compounds (Figure 1Av) [16][17][18]. Taken together, these properties lead to diverse mechanisms of Ni-catalyzed cross-coupling reactions, including two-electron and single-electron pathways mediated by Ni(0), Ni(I), Ni(II), and Ni(III) intermediates, and provide opportunities for achieving new products that are inaccessible with Pd ( Figure 1B).…”

Section: What Sets Nickel Apart?mentioning

confidence: 99%

Mechanisms of Nickel-Catalyzed Cross-Coupling Reactions

Diccianni

Diao

2019

Trends in Chemistry

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419

329

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“…In parallel with design and development of many new bidentate scaffolds, investigations of diimine-based catalysts have proceeded with unabated vigor. Such investigations have been driven by the subtle and intriguing mechanistic attributes of these systems, − the wide array of modified diimine structures available for study, and the ease of synthesis and activation of catalyst precursors, particularly the readily prepared diimineNiBr 2 precatalysts. There are numerous examples of modified nickel aryl diimine systems derived from incorporating new ortho -aryl substituents into the aryl diimine structures.…”

Section: Introductionmentioning

confidence: 99%

Ethylene Polymerization with Ni(II) Diimine Complexes Generated from 8-Halo-1-naphthylamines: The Role of Equilibrating Syn/Anti Diastereomers in Determining Polymer Properties

2019

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A series of 8-halonapthalen-1-amines (6a–d, X = F, Cl, Br, I) were prepared and converted to the α-diimines of 2,3-butanedione (7a–d). The nickel dibromide and zinc dibromide complexes of these diimines (3a–d and 8a–d, respectively) were obtained in good yields from standard precursors. NMR spectroscopic analysis of the zinc diimine complexes show the existence of syn and anti diastereomers with syn/anti ratios of ca. 2:1 (F), 2:1 (Cl), 1:1.5 (Br), and ca. 1:22 (I). Variable temperature NMR spectroscopy was used to calculate barriers to interconversion of these diastereomers which fall in the range 17–18.5 kcal/mol. Activation of nickel dibromide complexes 3a–d with modified methyl alumoxane (MMAO) yields cationic diimine complexes in which the 8-halo substituents lie over the axial coordination sites. Ethylene polymerization using these activated complexes is reported. The anti diastereomer of the diiodo catalyst (10d) in which both axial sites are blocked yields high molecular weight PE (ca. 106 g/mol) as the major fraction with a high turnover frequency at 40 °C. The minor syn diastereomer of 10d in which only one axial site is blocked produces low molecular weight PE as a minor fraction. PE formed from the dichloro catalyst 10b also exhibits a bimodal polymer distribution, but the high molecular weight fraction from the anti diastereomer is minor, while the syn diastereomer produces low molecular weight PE. The dibromo complex (10c) is unique in that interchange of diastereomers is on a time scale such that the PE produced shows a very broad molecular weight distribution (MWD ca. 14), spanning the range from the low to high molecular weight regimes. Catalyst 10a, bearing the small fluoro substituents, yields very low molecular weight PE (ca. 600 g/mol).

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Structural Characterization of β-Agostic Bonds in Pd-Catalyzed Polymerization

Cited by 29 publications

References 48 publications

Exploring Ethylene/Polar Vinyl Monomer Copolymerizations Using Ni and Pd α-Diimine Catalysts

Exploring Ethylene/Polar Vinyl Monomer Copolymerizations Using Ni and Pd α-Diimine Catalysts

Mechanisms of Nickel-Catalyzed Cross-Coupling Reactions

Ethylene Polymerization with Ni(II) Diimine Complexes Generated from 8-Halo-1-naphthylamines: The Role of Equilibrating Syn/Anti Diastereomers in Determining Polymer Properties

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